In certain manufacturing and other environments, particles in the air can be of sufficient particle size and proper combination to provide an undesirable combustive environment. Coal, sawdust, sugar, pollen, flour and other “dust” can ignite creating explosions resulting from very small particles with a high surface area to volume ratio (thereby making the collective or combined surface area of all the particles very large in comparison to a dust of larger particles) combine with air to create an explosive environment. Generally, dust is defined as powders with particles less than about 500 micrometers μm in diameter, but finer dust will present a much greater hazard than coarse particles by virtue of the larger total surface area of all the particles.
Dusts have a very large surface area compared to their mass. Since burning can only occur at the surface of a solid or liquid, where it can react with oxygen, this causes dusts to be much more flammable than bulk materials. For example, a 1 kg sphere of a material with a density of 1 g/cm3 would be about 27 cm across and have a surface area of 0.3 m2. However, if it was broken up into spherical dust particles 50 μm in diameter (about the size of flour particle), it would have a surface area of 1600 m2.
This greatly increased surface area allows the material to burn much faster, and the extremely small mass of each particle allows it to catch on fire with much less energy than the bulk material, as there is no heat loss to conduction within the material. When this mixture of fuel and air is ignited, especially in a confined space such as a filtration baghouse, warehouse or silo, a significant increase in pressure is created, often more than sufficient to demolish the structure. Even materials that are traditionally thought of as non-flammable, such as aluminum or iron, or slow burning, such as wood, can produce a powerful explosion when finely divided, and can be ignited by even a small spark. Such metal powders are widely used in fireworks for their dramatic effects.
Below a certain value, the lower explosive limit (LEL), there is simply insufficient dust to support the combustion at the rate required for an explosion. It has been determined that a number that is 20% lower than the LEL is considered safe. Similarly, if the fuel/air ratio increases above the upper explosive limit, there is insufficient oxidant to permit combustion to continue at the necessary rate. It is advantageous to prevent dust from reaching sufficient concentrations to avoid explosions.
For the explosion to occur, there has to be an ignition source. For example, static electricity is a major cause of fires and explosions in many industries. The hazard of electrostatic spark ignition of flammable vapor can be minimized by taking actions to limit the accumulation of electrostatic charges to safe values. Of primary importance is the proper bonding and grounding of equipment and containers. However, dust filtration, pollution-control filters, because of their large surface area, can generate as much as 200 times the electrostatic charge generated in the same piping system without filtration. Additionally, filters with conductive properties can be used to dissipate static electricity to reduce the ignition source for fires and explosions.
For example, United States Patent Application Publication 2011/0265312 is directed to a filter medium with improved conductivity. However, this reference uses a conductive coating which is disposed on the filtration substrate using a variety of techniques. For example, in one embodiment, the filtration substrate can be saturated with the conductive coating. In another embodiment, the conductive coating can be painted or extruded onto the filtration substrate. In another embodiment, the filter media can include a bonding agent that bonds the conductive coating to the filtration substrate. The bonding agent can be formed from, for example, a polymer such as polyvinylidene chloride, acrylic latex, polyurethane dispersion, polyvinyl acetate, polyvinyl alcohol, and combinations thereof. The bonding agent can also optionally be conductive. Each of these methods requires a process to add material to the underlying filter substrate. Further, the coating is just that, a coating, and can be worn off or otherwise separated from the filer substrate undermining the utility of this reference.
Another attempt to create a conductive filter is described in United States Patent Application Publication 2008/0230464 which includes a textile substrate (with a defined first side and a second side and a machine and cross-machine direction), where the conductive pattern on the first side is in registration with the conductive pattern on the second side of the textile substrate. However, this reference is limited to the use of two conductive patterns that must have a continuous conductive pathway between the two patterns and across the textile substrate.
Another attempt to produce for a conductive filter media is described in U.S. Pat. No. 5,527,569 which discloses a microporous filtration layer having electrically conductive particles embedded therein. According to this reference, a filtration layer is preferably formed of expanded polytetrafiuoroethylene (ePTFE) membrane filled with conductive particles and/or is coated on at least one side with a thin conductor. This reference, again, relies upon a coating for providing conductive properties.
U.S. Pat. No. 6,117,802 discloses nonwoven filter media that has a plurality of elongated generally hollow fibers, each having an internal cavity which has an opening, smaller than the cavity width, to the fiber surface and each retaining within the internal cavity an electrically conductive material. The electrically conductive material can be a large number of relatively small conductive solid particles. The small solid particles, which can be graphite, are permanently entrapped within the longitudinal cavities of the fibers without the use of an adhesive.
Accordingly, an object of the present invention is to provide a conductive filter media which has integrated conductive material for superior conductive properties without using a coating.
Another object of the present invention is to provide such a filter media that can be manufactured without a post-processing such as a coating.
Still another object of the present invention is to have a conductive filter with conductive pathways throughout the filter material regardless of planar direction.